This invention relates generally to the field of photoreproduction equipment and more particularly to an improved subject holder for holding the copy in multiple imaging camera/projectors.
U.S. Pat. Nos. 3,998,546 to Walley et al. and 4,582,406 to Wally disclose pre-press photoreproduction machines which are used to produce multiple image film flats or offset plates from either reflection copy, line transparencies or halftone films. This type of equipment is commonly referred to as a step and repeat camera/projector, and it often uses both front lighting for reflection copy and back lighting for images that have been previously exposed on film.
For various reasons, step and repeat camera/projectors have essentially replaced the prior procedure which involved hand "stripping" of small film negative segments. With modern photoreproduction machines, all of the image components are exposed to a single piece of film or a single offset plate, thus preventing the components from being out of registration as among the color separations used in halftone printing of multiple colors. Currently, high quality printing work of the most demanding character is performed with camera/projectors of this type, including the printing of books, magazines, labels, and a wide variety of advertising materials. The firms which use this type of equipment are among the largest and most competitive in the industry, and the speed with which the printing work is completed is highly important. Accuracy and quality are equally if not more important.
Machines of the type disclosed in the aforementioned patents, when equipped with state of the art lenses and lighting, make it possible for halftone work to be completed with retention of halftone dots at both ends of the scale; i.e., 1% dots in the shadow regions and 99% dots in the highlight areas. However, this high degree of accuracy in the photoreproduction process taxes the capabilities of the optical system, the chemistry, the film, the printing presses and the craftsmen who do the work. High quality equipment and superior workmanship are essential to achieving the desired results.
An important consideration with respect to achieving highly accurate photoreproduction is the relatively small depth of focus of the precision lenses that are used in the best equipment. At present, extremely high quality lenses have a depth of focus that is not more than 0.005 inch (plus or minus 0.0025 inch from optimum focus). Obtaining accuracy on this order is difficult even in small devices and is extremely difficult in large camera/projectors which can weight up to two tons and measure 14 or 15 feet long. For high quality work to be performed successfully, both the copy and the film that is to exposed must be held in a flat condition in planes which are parallel to each other and perpendicular to the optical axis with 0.005 inch. Thus, the mechanical features of the camera/projector must conform to an extremely high level of accuracy and geometric integrity or the small depth of focus of the lens will have a detrimental effect rather than a beneficial one.
The copy which is photographically reproduced is held by vacuum on a chase which may be opaque for reflection copy. However, when the copy is a transparency, the chase must have a clear glass center so that light can be projected through the chase and the transparent copy held on it. Typically, each camera/projector is equipped with several chases which are used alternately to hold different types and sizes of copy. For example, a chase used in the printing of lottery tickets would have a small glass panel at its center, while a longer yet somewhat narrow glass panel would be needed for printing canned food labels. A chase used for a double page advertising spread in an 81/2.times.11 inch magazine would have a glass panel in its center measuring about 12.times.18 inches so that an 11.times.17 inch film transparency could be accommodated with extra space provided at the edges to achieve a bleed trim where a picture or copy runs off the edge of the page.
Each chase is square but has a rectangular aperture which must be positioned at times with its long axis horizontal and at other times with its long axis vertical. For example, an 8 page film plate having each page 81/2.times.11 inches is made with each single page oriented vertically and each double page oriented horizontally, as shown in FIG. 7 of the accompanying drawings. In a 16 page film flat or plate, the single pages are horizontal and a double page spread is vertical, as shown in FIG. 6.
The chase is normally constructed from aluminum plate which is milled on both faces. Even when the milling and finishing operations are carried out properly, it is impossible to maintain the faces of the chase perfectly flat. Moreover, the aperture in the center of the chase detracts from the structural integrity of the chase and can create warping and other distortions. A glass panel about 1/4 inch thick is normally applied to the aperture by seating it on a shoulder around the aperture and holding it in place with liquid silicone rubber or another glazing material. The thin glass pane is likely to be thinner at some places than at others, and the bed of silicone rubber is inevitably thicker at some areas than at other areas. The result of these imperfections is that the glass and the face of the chase are not parallel.
These problems are compounded by the way in which the chase has traditionally be held on the chase carrier. Eight bolts are usually spaced arounmd the edge of the chase carrier and are threaded into the chase to hold the chase against the face of the chase carrier and thus achieve a reasonable degree of flatness. However, the face of the chase carrier is not exactly flat and is not necessarily perpendicular to the axis of the optical system. Also, the chase is not flat and the glass is not parallel to its face in any event. Thus, when the chase is in place, the glass panel can be displaced considerably from a perpendicular relationship to the optical axis, and this condition can create substantial inaccuracies, particularly with high quality lenses. If the glass panel which receives the copy is more than 0.0025 inch away from a perpendicular relationship to the optical axis, the short depth of focus of the lens can actually have a detrimental effect and the quality of the finished work suffers accordingly.
This is true if the camera operator carefully applies the mounting bolts. The accuracy is even worse if the bolts are applied improperly or if less than all of them are used. Often, the camera operator is under considerable pressure to quickly finish a high volume of work. By way of example, it is not uncommon for book or magazine pages to be shot at rates up to six per minute, and the chase must be replaced many times and rotated 90.degree. many more times during the course of a day's work. With this type of time pressure on the camera operator, it is highly tempting for him to attempt to "short cut" the nearly five minutes that are required to remove all eight bolts and then thread them back in after the chase has been replaced or rotated. Even if the camera operator properly uses the bolts, valuable time is required each time the chase is replaced or rotated.
The time pressure under which camera operators often work leads them to use less than all of the bolts, and many operators apply only four bolts as a matter of course. Since four bolts are unable to secure the entire periphery of the chase against the chase carrier, the chase is not held properly and can warp rather severely. Then, the depth of focus of the lens is exceeded by the offset and warpage of the plane of the copy that is to be shot, and the dot retention in the film can be corrupted well beyond acceptable limits.